The indications for a spondylodesis (stiffening) of the entire spinal column or adjacent vertebrae are very complex. Stabilizations, for example, are performed in case of degenerative change in the spinal column, after tumor removal, infections, or trauma.
In many cases, combinations of dorsal systems (e.g. rod-pedicle-screw systems) combined with multiple cages for ventral bracing are used for stabilizing the spinal column.
We know from biomechanics that the statics of the spinal column, as weight-bearing unit, can be compared with those of a crane. The anterior column, which consists of vertebrae and intervertebral discs, bears about 80% of the weight, the dorsal structures bear approx. 20%. From this we can deduct that the anterior portion of the lumbar spine is primarily subject to compressive forces, while the dorsal portion is primarily subject to tractive forces. In addition, shearing, torsional, and bending forces act on both columns.
While the above-mentioned screws/rod-systems are used in the dorsal area, a cage or implant is used for ventral bracing.
The goal of the stabilization of the vertebrae is a quick ossification of the intervertebral space, in order to achieve lasting freedom from pain for the patient.
Examples of the indication of a spondylodesis are: unstable vertebral fractures, degenerative instabilities, fractures with sufficient anterior bracing, dislocations, spinal tumors (without anterior defects), prior failed fusion (pseudoarthrosis).
The surgical techniques are carried out in open fashion or as minimally invasive procedures, depending on the indication. These procedures usually differ in size and type of access or accesses.
Accesses to the affected area can be gained through a combination of a ventral and dorsal access (in case of the cervical spine sometimes ventral only) or through a dorsal, dorsolateral, or lateral access.
An implant of the appropriate dimensions is chosen, depending on the structures and proportions found. Generally, the size is calculated in such a fashion that the screw reaches into the anterior third of the vertebral body.
Pedicle screws are screwed into the vertebral body through the pedicle. For safe navigation and guidance, some screws are cannulated and can be inserted through a guide wire. Some have additional cross-holes in the thread piece for subsequent cementing.
A rod, which connects two or more pedicle screw with each other, is inserted into the head of the screw, also called “tulip”. The tulip can be rigid or movable in one or multiple directions, to facilitate the later insertion of the rod.
After the rod has been inserted on both sides, the vertebral bodies to be fixated are pressed apart with varying spreading systems, generally via the pedicle screws, to achieve the optimal distance between the vertebral bodies.
During this process, the mobility of the tulip can be obstructive if the objective is to raise vertebral bodies or reconstruct a physiological alignment (mainly in accident surgery).
Minimally invasive procedures require an extension of the tulip, to allow for the extracorporeal, percutaneous insertion of the rod.
Depending on the surgical technique, a variety of a manufacturer's screw forms and associated instrument sets are used.
It is disadvantageous that these consist of a variety of instruments because only experienced surgeons and surgical assistants can use this set of instruments without the manufacturing companies' product manager.